HitchHike Tag Bums a Ride from Wi-Fi Access Points

A system being developed by Stanford University researchers could enable a low-cost Wi-Fi tag to receive transmissions from access points, and to use the energy of those transmissions to send its own unique signal to a laptop or mobile device.

A team of Stanford University researchers have developed a small Wi-Fi device that behaves similarly to a radio frequency identification tag, sending a unique identifier that indicates where it is located, but doing so via Wi-Fi access points without impacting the Wi-Fi network where the tag is located. The solution, known as HitchHike, consists of a small tag—which could cost the same as an ultrahigh-frequency (UHF) RFID tag—as well as standard Wi-Fi access points and a smartphone, tablet or laptop. The HitchHike tag captures a transmission from a Wi-Fi access point, and then, in turn, transmits its own Wi-Fi signal to a receiver—a mobile phone, laptop or other Wi-Fi-enabled device. In that way, the tags do not interfere with other traffic on the Wi-Fi network.

The HitchHike tag was developed to enable a low-cost, wireless transmission of data, says Pengyu Zhang, a Stanford University postdoctoral researcher for a research team led by Sachin Katti, an associate professor of electrical engineering and computer science. RFID systems can capture tag ID numbers and sensor data, but they require the installation of fixed or handheld readers. Using the Wi-Fi network to transmit data can create a drag on that network due to the additional traffic. The solution that the Stanford researchers developed is a Wi-Fi tag that uses the network only to energize and prompt a transmission, which the device can then alter and send to a dedicated phone, laptop or tablet.

Stanford University's Pengyu Zhang

The tag, approximately the size of a postage stamp, contains an RF switch that reflects the incoming excitation signal to the receiver, a chip and an envelope detector that identifies when the incoming excitation Wi-Fi signal starts. It could also come with a power source connected to it if sensor data were being collected and transmitted.

The chip captures incoming Wi-Fi signals as a series of ones and zeros, then translates that code to its own set of data. To avoid interfering with the original signal from the access points, the chip shifts the new stream of data and forwards it to a different Wi-Fi channel. This prevents collision of data that would otherwise occur in a wide-scale Wi-Fi-enabled device deployment. "The HitchHike tag uses an ultra-low-power circuit to detect the starting point of an ambient Wi-Fi transmission," Zhang says. Once it detects a Wi-Fi signal, such as that from standard access points or routers in businesses or homes, it "reflects" its own ID in another Wi-Fi signal. The frequency-shifted transmission to a receiver could also impact traffic on a Wi-Fi channel, Zhang adds. However, his group is developing a media access control (MAC) layer protocol to solve that problem.

That signal reflection, Zhang explains, uses a technique known as codeword translation, by which it injects its own information by modifying the signal received from the access point. "The phase modification is performed in a way such that the reflected signal is still a valid Wi-Fi signal," he states. "Therefore, we can use commercial Wi-Fi receivers—such as that built into a phone or tablet—to decode the reflected signal."

The tag can be either passive or active, depending on the requirements of a particular use case. If, for instance, a user wanted merely to identify what tags were at a specific location, the tags would not require a battery. Like RFID tags, each HitchHike device is encoded with its own unique identifier, and that can be captured by a phone or a laptop's Wi-Fi device when the tag is energized by a Wi-Fi access point. The HitchHike tag radio consumes 33uW only, enabling it to be energized via a Wi-Fi connection.